With the fast development of the optical communication technology, the optical communication system develops in the direction of high-speed, high-capacity, long-distance and intelligence, etc. Currently, the DWDM (Dense Wavelength Division Multiplexing) optical communication system with 160 channels and 10 Gb/s single channel speed is already commercially applied, as well as the ASON (Automatically Switched Optical Network) system based on intelligent circuit-switching. In the future, there will be an optical network system with much larger capacity and much higher speed (40 Gb/s or above), meanwhile there will also appear intelligent optical network system with functions like OXC (Optical Cross Connect) or ROADM (Reconfigurable Optical Add/Drop Multiplexer).
In the super high speed optical transmission system, dispersion and PMD (Polarization Mode Dispersion) will deteriorate the optical signal quality and reduced the transmission distance. In order to expand the transmission distance of the optical transmission system, some measures need to be taken to compensate dispersion and PMD. In the traditional optical communication system, it mostly adopts the fixed dispersion compensators to compensate the dispersion of the optical transmission path. FIG. 1 is a diagram of the dispersion compensation method for the traditional optical communication system. By adopting dispersion compensation, it ensures that the residual dispersion of the optical signals at the receiver can be controlled within the range of which the receiver can tolerate. The dispersion compensator is generally realized by using dispersion compensation fiber module. The dispersion compensation module can be placed in the optical terminal station or optical regeneration station. In the optical terminal station, the dispersion compensation module is often used to perform dispersion compensation function for the multi-channel optical signals of DWDM system after multiplexing or before de-multiplexing. In the optical regeneration station, it usually adopts two-stage optical amplifier structure and a dispersion compensation fiber module can be placed between the two stage amplifiers to realize multi-channel dispersion compensation.
For the long-distance optical transmission system, factors such as temperature, pressure and the like will cause tiny changes of the dispersion parameter of the link fiber. As a result, the real-time changes of the optical signal dispersion value in the optical path accumulates as the distance of the total transmission increases, which will probably lead to the result that the signal residual dispersion at the receiving end goes beyond the tolerance range and deteriorates the system bit error performance.
With the increase of the single channel transmission speed, the dispersion accommodation of the optical source will decrease. For the optical signals at the speed of 10 Gb/s without chirp, the dispersion accommodation is about 1000 ps/nm; however for the optical signals at the speed of 40 Gb/s without chirp, the dispersion accommodation is about 40 ps/nm, which is only equivalent to the 2 km transmission distance of the 1550 nm window of G.652 fiber. Therefore, for the 40 Gb/s system, since the source dispersion accommodation is relatively small, any tiny changes of the link fiber dispersion value will cause the signal residual dispersion at the receiving end to go beyond the tolerance range, thereby deteriorating the system bit error performance. So influence of the change of the optical fiber dispersion on the 40 Gb/s system performance is particularly obvious.
On the other hand, with the development of the intelligent optical networks, the ROADM and even the OXC nodes will be incorporated into the optical network. In these optical network nodes, the dynamic add-drop multiplexing of the optical signals as well as the dynamic optical cross-connect will cause changes of the dispersion value of the optical signals in the optical path, which will cause the signal residual dispersion at the receiving end to go beyond the tolerance range and deteriorate the system bit error performance.
In summary, the above factors indicate that with the optical communication system developing in the direction of high-speed, high-capacity, long-distance and intelligence, and after the optical signals are transmitted in the link, the signal residual dispersion value at the receiving end will constantly change because of the dispersion value changes of the link and the optical network nodes. It is necessary to perform adaptive dispersion compensation for the signals that have constantly changeable residual dispersion value. How to realize the adaptive dispersion compensation is a key question in the present technology area.
The adaptive dispersion compensation of the optical transmission system could be realized by using the tunable dispersion compensator which could be realized either in optical field or in the electric field.
There are many methods for realizing electric dispersion compensation, as shown in FIG. 2 which is a schematic diagram of the receiver with electric dispersion compensation function. After photoelectric conversion and linear amplification, the optical signals are transmitted to equalization circuit; by checking the electrical signal quality after equalizing, taking certain controlling measures and using the performance of the feedback control equalizer, adaptive dispersion compensation can be realized. The characteristic of this method is: easy realization of the method, fast response speed, but small dispersion compensation range; it could only realize the single channel dispersion compensation. The equalization circuits used for electric dispersion compensation could be FFE (Feed-forward Equalizer), DFE (Decision Feedback Equalizer), FDTS (Fixed Delay Tree Search) or MLSE (Maximum Likelihood Sequence Estimation), or a combination thereof. The electric dispersion compensation technology has smaller compensation dispersion value, for example, for the 10 Gb/s signal, the electric dispersion compensation is equivalent to adding 20-40 km transmission distance in the G.652 fiber.
FIG. 3. is a schematic diagram of the optical tunable dispersion compensator in realizing adaptive dispersion compensation. Adaptive dispersion compensation function can be realized by checking the system residual dispersion or detecting the system bit error performance as well as controlling the dispersion compensation value of control tunable dispersion compensator through feedback. There are many methods in realizing optical tunable dispersion compensation, including CFBG (Chirped Fiber Bragg Grating) technology, G-T (Gires-Toumois) based interference technology, MEMS (Micro Electro Mechanical Systems) based technology, PLC (Planner Lightwave Circuit) circular resonator technology and multi-stage DCM (Dispersion Compensation Module) cascade, etc.
Different dispersion technologies have different response time for compensating dispersion. As shown in FIG. 4, it lists the response time of several tunable dispersion compensators, in which the EDC (Electric Dispersion Compensation) has the shortest response time, almost reaching the order of magnitude below millisecond; while the optical tunable dispersion compensator has longer time for adjusting, from tens of millisecond to tens of seconds; VIPA (Virtual Imaged Phased Arrays) technology has the longest response time, which almost reaches the order of magnitude of second.
The performance detected by the adaptive dispersion compensator could be the system bit error; this dispersion compensation method needs to adjust through feedback multiple rounds based on the bit error performance. Considering the fact that the single round response time of the optical tunable dispersion compensator is relatively longer when compared to EDC, thus the total response time will be longer.
The performance detected by the adaptive dispersion compensator can also be the residual dispersion or the shape of the optical signal. However, the technology in the dispersion compensation method for realizing the detection for residual dispersion and optical signal shape is complicated, thus it is hard to realize. Among these methods, the detection precision and the detection time of residual dispersion will directly affect the performance of the adaptive dispersion compensator; what's more, the adjusting process of the adaptive dispersion compensation usually requires several rounds of adjustments, which significantly increases the time for adjusting of dispersion.
It can be seen that if the adaptive dispersion compensation is realized by directly detecting the residual dispersion, the non-linear effect, the PMD effect (Polarization Mode Dispersion) and the like will directly affect the dispersion detection precision; while the indirect dispersion detection method by using error detection often requires several rounds of adjustments before completion, thus the response time is relatively longer. The pure electric dispersion compensation technology generally has smaller compensation range.